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MPC5746R-252DS borderline design (VDD_LV_CORE)

Question asked by James Murray on Aug 22, 2019
Latest reply on Oct 9, 2019 by James Murray

Seems that as supplied, the power supply capacitance design on the MPC5746R 252 daughtercard is borderline.

 

Test procedure - set the board to use the MB 1.25_SR as the source for the core:

Daughterboard:

J19 = open

J17 = open

J18 = 1-5

 

MPC57XXMB Motherboard:

J60 = open

 

#1

Power up.

Processor boots ok.

 

#2 Now replace the J18 shunt with a length of wire, say 70mm long.

Power up.

Processor is stuck in reset.

 

PORST is off. Scoping the RESET line shows a 50% duty 121Hz signal.

 

#3

Add a 4.7u capacitor between TP16 and TP18

Power up.

Processor boots ok.

 

The schem shows a number of non-pop 4.7u caps on VDD_LV_CORE - why ? I suggest these be populated in future builds.

 

Test #2 is similar to using a off-board 1.25 regulator.

 

regards

 

James

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